test: migrate examples to XXXX-category-name layout + split expected streams

Rename all example tests/companions to the XXXX-category-test-name scheme
(per-category 100-blocks: basic 0010, types 0100, ... errors 1000,
diagnostics 1100, ffi 1200, ffi-objc 1300, ffi-jni 1400, vectors 1500,
platform 1600). Companions and dir/C fixtures move in lockstep with their
parent test; #import/#source/#include paths rewritten to match.

Expected output now lives in examples/expected/ (a sibling dir of the
tests) split into three streams per the new convention:
  <name>.exit / <name>.stdout / <name>.stderr  (+ optional <name>.ir)

run_examples.sh rewritten: scans examples/ and issues/ for an
expected/<name>.exit marker, captures stdout and stderr separately (no
more 2>&1), compares each stream + exit + optional IR snapshot.

Behavior validated unchanged: every renamed test reproduces its prior
merged output + exit (diffs limited to file paths/basenames embedded in
diagnostics + traces, which correctly reflect the new names). Suite:
292 passed, 0 failed. 50-smoke.sx split + issue relocation + docs follow
in subsequent commits.

examples/1211-ffi-03-large-struct.h

@@ -0,0 +1,22 @@
+// FFI large-struct (>16 B) by-value roundtrips. These route through
+// the byval-pointer ABI path (caller copies onto its stack, hands the
+// callee a pointer; on AAPCS64 a separate `x8` indirect-return
+// register; on SysV AMD64 a hidden first arg). Distinct from the
+// register-pair / [2 x i64] / HFA paths the small-struct baseline
+// covers — locking those in here keeps the byval path honest.
+//
+//   Big24 — 24 B, three s64
+//   Big48 — 48 B, six s64
+
+typedef struct { long long a; long long b; long long c; }                 Big24;
+typedef struct { long long a; long long b; long long c;
+                 long long d; long long e; long long f; }                 Big48;
+
+Big24     ffi_big24_make  (long long a, long long b, long long c);
+Big24     ffi_big24_rotate(Big24 v);
+long long ffi_big24_sum   (Big24 v);
+
+Big48     ffi_big48_make  (long long a, long long b, long long c,
+                           long long d, long long e, long long f);
+Big48     ffi_big48_reverse(Big48 v);
+long long ffi_big48_sum   (Big48 v);